Metal coated abrasives such as diamond and cubic boron nitride have been used in resin bonded abrasive tools for many years, since the discovery that abrasive tools containing only these specific abrasives are uniquely beneficiated when the metal coatings are relatively thick and continuous, the usual thickness being about 1/10th to 1/50th of the base particle diameter. When used in grinding wheels the performance is enhanced from two to five fold as measured by longer wheel life, see e.g. U.S. Pat. Nos. 3,645,706 and 3,957,461.
This increased performance results from a combination of a number of factors such as: (a) more uniform dispersion of the abrasive in the matrix, (b) improved chemical bondability, (c) improved mechanical bonding, (d) a mechanical shell holding cracked grains together, (e) a high melting point, hard layer which absorbs thermal energy generated in grinding which would soften and degrade the resin and other factors.
The most common method of metal coating these electrically non-conducting abrasives is by the electroless plating process. In a typical process the surface of the abrasive is "activated", i.e. by deposition of spots of palladium on the surface through the decomposition of a palladium salt (e. g. Palladium Chloride). Prior to this activation step, the crystal surface may be sensitized by agitating the crystals in a heated bath of stannous chloride. U.S. Pat. No. 3,556,839 teaches a continuous process for coating diamonds with nickel by the electroless process. A batch process for accomplishing the same thing is explained in Example 3 of U.S. Pat. No. 3,904,391.
A good general discussion of electroless plating is found in Lowenheim, F. A., Electroplating, McGraw-Hill Book Co., 1978, Chapter 17. Through Lowenheim and through two previously mentioned U.S. Pat. Nos. (3,556,839 and 3,904,391 which are incorporated by reference herein) several different nickel plating bath compositions are taught along with the appropriate conditions. The bath temperatures seem to range between 57.degree. and 95.degree. C., and pH ranges from 4 to 11. If a batch type process is used, treatment of the crystals in one plating bath batch may be insufficient, after depletion of the metal content, to obtain the desired coating thickness. Therefore, a series of sequential batch steps called stations (as many as 10 to 30) may be necessary. In normal practice, each coating or plating bath station is depleted to about 80 to 85 percent of its metal content (taking about 20 minutes) at which time the bath is emptied of liquid and a fresh coating bath started. However, the activation step (e. g. addition of palladium chloride) need only be performed in the initial bath. Hydrogen gas is evolved during the process, and therefore, adequate exhaust ventilation is required. Sufficient agitation is required during the coating to prevent crystal agglomeration.
It has been found that metal coatings of increased exterior surface roughness lead to improved abrasive tool performance, probably because of the increased area for mechanical and/or chemical bonding. Inherent in the electroless metal coating process is that, as it is normally practiced, the surface tends to get smoother as the metal layer is built up (see U.K. Pat. No. 980,030, p. 4, lines 23-26), hence the desired roughness of the final surface is not achieved.
One method to roughen the surface of metal coated diamond is given in U.S. Pat. No. 3,650,714. This patent proposes adding ceramic whiskers during the coating of diamond with copper or nickel and also obtaining roughness by heating a mixture of sponge iron and braze coated diamond under a vacuum.
Irish patent No. 21,637 teaches a process for electroless nickel coating of a nonmetallic body wherein the surface of said body is physically roughened in order to help secure the activating palladium salt.
The object of this invention is to create a rough surface metal coated crystal without having to resort to the techniques of the last two mentioned patents.